Question: 3 folders cost $5.79. Which equation would help determine the cost of 13 folders?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 3 folders. We want to know the cost of 13 folders. We can write the numbers of folders as a proportion: $\dfrac{3}{13}$ We know 3 folders costs $5.79. We can let $x$ represent the unknown cost of 13 folders. The proportion of these costs can be expressed as: $\dfrac{\$5.79}{x}$ The cost changes along with the number of folders purchased, and so the two proportions are equivalent. $\dfrac{3}{13} = \dfrac{\$5.79}{x}$